Recent advancements in the field of cellular communication include supporting direct or device-to-device (D2D) communication between two or more mobile devices operating in the licensed spectrum. This has largely been motivated by the popularity of proximity based services steered by public safety needs as well as social networking applications trends. By enabling direct communication, D2D capable user equipments (D2D-UEs) are able to exploit their proximity with each other to communicate under network coverage and/or out of network coverage. As such, D2D-UEs are able experience reliable, high quality and versatile voice and data communication, and high speed data rates. D2D communication may also increase a capacity of a cellular network due to traffic offloading.
In mission-critical communications (MCC), users generally need to be able to communicate everywhere at any time, regardless of lack of network coverage, lack of resources, and in case of any type of network failure. This is particularly the case in emergency communications.
FIG. 1 illustrates a chart 100 of Mission Critical Communication Modes vs Applications for D2D communication. In particular, several D2D operation modes 110, including direct mode 120, repeater mode 130, network gateway (or relay) mode 140, and multi-watch or multi-channels monitoring/listening mode 150 are illustrated with reference to a plurality of applications 160, including voice 161, user messages/data 162, regular status reports (e.g. position reports or periodic check-in) 163 and/or high speed data 164.
In MCC, D2D communication can be used to provide a private call directly with another user, as illustrated in system 121 of FIG. 2, or to provide group calls with several other users, as illustrated in system 125 of FIG. 3. In both cases, users are within proximity of each other, but may be with or without network coverage or network infrastructure support. The call may be voice call, short burst data call or high speed data call.
Furthermore, a repeater D2D terminal 135 can be used to enlarge D2D coverage 136 when needed, as illustrated with reference to system 131 of FIG. 4, enabling a first D2D terminal 132 to communicate with a second D2D terminal 133 even though the first and second D2D terminals 132, 133 are out of range with each other due to distance separation or shadowing (i.e. shadow fading). This is possible as both the first D2D terminal 132 and the second D2D Terminal 133 are within the transmission range of the D2D repeater 135.
Yet further again, as illustrated with reference to system 141 in FIG. 5, a gateway D2D terminal 142 that is in a coverage area of a network (144) can be configured to provide network connectivity services for an out of network coverage D2D terminal 143 to communicate with its management centre 145, or with another D2D terminal 146 that is in the same communication group, or even with different communication groups but is not in the local area.
Finally, as illustrated with reference to system 151 of FIG. 6, a mobile command unit D2D terminal 152 may be configured to monitor/listen on air interfaces 153, 155, 157 to more than one D2D group communication 154, 156 and cellular network in a geographical overlapping local area 159. The mobile command unit D2D terminal 152 may have multi-watch or multi-channel monitoring/listening capability that may also have network gateway capability to provide connectivity to surrounding D2D terminal to communicate to a management centre 160 and enable communication among different public safety agencies operating in the same local area.
The 3rd Generation Partnership Project (3GPP) has been working towards including D2D communication as an overlay to its existing Long Term Evolution (LTE) cellular network, with the target of bringing support for public safety communication into its upcoming Release 12 and future Release 13. Mission Critical (MC) Push-To-Talk (PTT) functionality, similar to that provided by Professional Mobile Radio (PMR) and Land Mobile Radio (LMR) systems, has been prioritised for inclusion into the upcoming Releases. MC-PTT primarily targets group call support, but private or individual one-to-one calls will also be supported. The 3GPP specified MC-PTT is likely to work in both in-network and out-of-network scenarios, but it is generally understood that not all functions will be available when a device operates out-of network.
3GPP MC-PTT is anticipated to include regular group calls and broadcast group calls, as illustrated in the system 200 in FIG. 7A and FIG. 7B, where no response is expected by the initiating user equipment 221 and groups calls based on priorities such as an emergency group call may pre-empt other calls in progress. Private or individual one-to-one calls, where the call can be heard by other users may also be supported. Calls (i.e. voice call or user data call) initiated by a user may happen in an ad-hoc/random fashion, where each call starts with selecting a Scheduling Assignment (SA) resource index randomly for SA (225) transmission followed by associated MAC-PDUs transmission (226) resulting in a half-duplex communication within an SA period.
Technically, the anticipated 3GPP MC-PTT system is sufficient for voice and user data communication in the basic direct or D2D mode (corresponding to 121 and 125 of FIG. 2 and FIG. 3) where each call arrives as random events. However, the anticipated 3GPP MC-PTT system is unable to provide repeater functionality, such as illustrated in 131 of FIG. 4, network relay functionality, as illustrated in 141 of FIG. 5, or multi-watch or multi-channel monitoring/listening functionality, as illustrated in 151 in FIG. 6.
In these cases, a centralised/field device or network relay device would experience a large amount of messages that are periodically sent from the devices that are attached to it causing interference/collision at its receiver. For example, with reference to system 300 of FIG. 8, when periodic position reporting or periodic system check-in for lone field personnel is used, collision and/or interference occurs at the D2D terminal receiver 301 as D2D Terminal 2 302, D2D Terminal 3 303 to D2D Terminal k 304 may independently, simultaneously and periodically transmit data to the field control D2D Terminal 1 301. That can result in the loss of a position information report or failure of a system check-in from other D2D-UEs transmitters, such as D2D terminals 302, 303, 304.
As such, there is a need for an improved method and system for D2D communication.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.